1. Field of the Invention
The present invention relates to an optical medium such as optical recording media with which the recording of information or the playback of information recorded on an optical disk or the like can be performed.
2. Description of the Related Art
In an effort to increase recording capacity, a new type of optical disk called a bonded optical disk has been proposed in recent years, in which two disk substrates are used, each comprising a recording layer formed on a transparent resin substrate, and these two disk substrates are bonded with an adhesive layer sandwiched therebetween.
Hot-melt adhesives and radical polymerizable UV-curing adhesives, for example, have been used as the material for the adhesive layer that holds the disk substrates together in these bonded optical disks, but these could not be considered practical because of their poor heat resistance and warping of the disk substrates.
In view of this, the use of a liquid cationic polymerizable UV-curing adhesive has been proposed, as discussed in, for instance, Japanese Laid-Open Patent Application H7-126577. These cationic polymerizable UV-curing adhesives are expected to provide a huge improvement to the manufacturing process because the curing reaction proceeds upon irradiation with UV rays even in the air or in the dark, and because of the less shrinkage during the reaction, other benefits are less warping of the disk substrates and sufficient adhesive strength after curing.
Thereupon, the applicant proposed adding a so-called ion catcher (or ion exchanger) to a resin composition in an effort to prevent corrosion of the recording layer in high temperature and humidity environments when one of these cationic polymerizable UV-curing adhesives is used (Japanese Patent Application H8-59555).
This addition of an ion catcher allows ionic substances in the adhesive layer to be trapped, so the effect is better corrosion resistance.
However, because of the high light-proof property of the ion catcher, the UV rays cannot pass sufficiently into the interior of a UV-curing resin if the ion catcher is added in too large an amount, so the reaction efficiency is poor in the curing of the adhesive agent.
In order to solve the above problems, the present invention provides an optical medium with better corrosion resistance, which is accomplished by using an adhesive agent that has no detrimental effect on the recording layer in an optical medium.
The optical medium of the present invention has a recording layer and an adhesive layer between at least two substrates, wherein the adhesive layer contains (A) a cationic polymerizable resin, (B) a photo-initiator, and (C) an anion catcher.
In another aspect of the present invention, the weight ratio of the photo-initiator (B) and the anion catcher (C) in the above-mentioned optical medium is from 3:1 to 3:24.
In another aspect of the present invention, the weight ratio of the photo-initiator (B) and the anion catcher (C) in the above-mentioned optical medium is from 3:1 to 3:12.
With the above structure of the present invention, because the adhesive layer contains a cationic polymerizable resin (A) and a photo-initiator (B), the curing reaction proceeds upon irradiation with UV rays or other light, and there is also less shrinkage during the reaction.
Furthermore, because the anion catcher (C) is contained, chlorine ions and other ions contained in the resin can be removed more effectively, and this suppresses the corrosion of a reflective layer or recording layer by these ions.
Corrosion of a reflective layer or recording layer of the optical medium by ions can be suppressed particularly well when the weight ratio of the photo-initiator (B) and the anion catcher (C) is from 3:1 to 3:24.
Moreover, if the weight ratio of the photo-initiator (B) and the anion catcher (C) is from 3:1 to 3:12, an adhesive layer with good reactivity will be obtained, so this optical medium can be produced more efficiently in the manufacturing process.